Differentials traditionally have been used for distributing power between the wheels on opposite sides of a vehicle in order to compensate for different relative rates of rotation during maneuvering. The differentials of the prior art typically include a ring gear driven by a pinion gear mounted on the drive shaft. The ring gear is secured to a differential case or housing for rotation therewith. The axles are coupled to coaxial bevel gears which mesh at right angles with pinions mounted on spindles within the differential case. When traveling straight ahead, the differential case simply rotates with the ring gear and there is no relative motion between the pinion and the bevel gears therein. During maneuvering or when rounding a curve, however, one wheel must travel relatively faster and the difference is compensated for by the pinion gears which permit opposite relative rotation of the bevel gears within the differential case so that faster rotation of one axle and wheel is offset by proportionately slower rotation of the other axle and corresponding wheel.
Vehicles, such as trucks and semitractor trailer rigs utilizing tandem axles and dual wheel assemblies, present a more complicated problem. If the axles are arranged in tandem and are both driven, some means must be provided for transferring power between the axles as well as between the dual wheel assemblies on opposite ends of each axle. Various such interaxle differentials have been developed for this purpose. For example, see U.S. Pat. No. 3,590,954 to Plantan. It will be appreciated that the wheels in each wheel assembly of tandem driven axles are usually secured together and driven in common on the same axle. Although the need for compensation is greatest between the dual wheel assemblies on opposite sides of the vehicle, some compensation is also needed between the wheels in each assembly. That is particularly noticeable during tight maneuvering, when tire scuffing and hopping can occur, which in turn decrease control and increase tire wear. The proposed solutions of the prior art in this regard have not been satisfactory. For example, U.S. Pat. No. 2,727,582 to Lisenby shows a differential drive for dual wheel assemblies wherein the inner wheel can be selectively fastened in driving relationship with the outer wheel of the assembly. U.S. Pat. No. 2,126,960 to Higbee shows a complicated independent dual wheel drive which comprises a primary differential and two secondary differentials, one for each of the opposite dual wheel assemblies.
A need has thus arisen for an interaxle differential of improved construction which not only compensates for differential rotation between opposite dual wheel assemblies, but which also compensates for differential driven rotation of the wheels in each assembly.